Monoclonal antibody 33.1, prepared against a human EBV-transformed B cell line has been characterized in detail. The antigen 33.1 is a human homologue of the I-A murine class II MHC antigen. The antigen has been characterized by immunofluorescene, flow cytometry, electrophoresis and primary structure analysis using radiosequence methodology. The results show that 33.1 is an I-A homologue but that it is distinct from the previously reported I-A homologues MB and DS. Unlike these antigens, 33.1 is virtually absent on resting B cells but strongly positive on activated B cells. It is very similar to MB in structure but can be distinguished by a few residues near the N-terminus. Rabbit-mouse hybridoma H89 secretes and allotype-deficient rabbit H chain which nonetheless has all of the structural features of a H chain from the a-positive subgroup. Monoclonal antibodies have been prepared against human IgE.